The present invention relates to a device for combustion of moist fuel in which a fire grate, on top of which the fuel burns, is mounted inside a combustion chamber. The mechanical arrangement is also provided with a fuel-feeding apparatus and a discharge channel for removing flue gas from the combustion chamber.
The humidity of the organic fuel such as wood chips and turf or the like used in combustion plants is generally too high for direct feeding from the storage into the combustion device. Such fuels are first dried in a separate desiccator and then fed into the combustion device using a separate feeding apparatus. A separate desiccator is expensive and consumes energy. Desiccation, as well as transfer to and away from the desiccator are all separate stages of the process, and each of them increases the processing expenses of the fuel.
In addition, the currently available combustion devices share the problem that even though the fuel is desiccated, it is still so damp that a long fire grate is necessary for successful combustion. The fuel is shifted along the grate, desiccating at the beginning and burning further away on the grate. A long grate requires a long combustion chamber, increasing the costs of the combustion device. The combustion process in large thermal power stations which use moist chip fuel, even up to 65 per cent in humidity, is based on long fire grates and ceramic masonry, enabling efficient evaporation, desiccation and combustion of the fuel. In order to burn properly, the fuel for currently existing small-sized boilers, front end furnaces and stokers must not exceed 35 per cent in humidity.
The purpose of the invention is to introduce a device for combustion of moist fuel, at the same time remedying defects of the currently available devices. Primarily, the purpose is to introduce a device with which no separate desiccator is needed and in which the fuel can be fed into the combustion chamber moister than in the currently available small-sized boilers or similar devices. Further, the purpose is to introduce a device in which the fuel does not need as many transferring stages as compared to the currently available devices, and, in which the combustion chamber and the fire grate are relatively short.
The purpose of the invention will be achieved with a device which is characterized in what has been stated in the patent claims.
The device constructed in accordance with the invention is provided with a drum dryer which is attached to a feeding apparatus and from which fuel is fed directly onto the grate. When the drum dryer is mounted inside the combustion chamber, the temperature inside the dryer is so high that the fuel fed in it dries effectively before it drops onto the grate. Thus, no separate dryer is needed, and the fuel need not be separately fed into nor removed from the dryer. Fuel processing costs will thus reduce and energy will be saved. Furthermore, the fuel dropping onto the fire grate will be so dry that the grate is no longer needed for desiccating the fuel, thus making it possible to use a relatively short grate. The combustion chamber can efficiently use fuel the humidity of which is approximately in the range of 35 to 55 per cent. One advantage is also that the device collects and utilizes the evaporation energy used in desiccating the fuel, increasing the efficiency of the device.
In an inexpensive application of the device the drum dryer is provided with an inner part with an involuted spiral directed towards the combustion chamber. The involuted structure allows unobstructed advancing movement of the fuel. At the same time, the fuel is constantly mixing and desiccating very efficiently. The transition distance of the fuel is .pi. times the internal diameter of the drum times the number of rising spirals, thus making the transition distance very long even in a relatively short drum dryer.
In another inexpensive further application the device is provided with external bearers for the drum dryer and with an actuator for running the dryer. When mounted outside the combustion chamber, the apparatuses are easy to service.
In accordance with the invention, the fire grate is mounted in a diagonal position, and the end of the drum dryer mounted in the combustion chamber reaches a position right above the upper end of the fire grate. In this arrangement the fuel will shift downwards from the position where it is being fed, thus allowing secure forward movement of the fuel on the grate and as well as constant combustion.
In addition, one application of the invention is provided with a fire grate consisting of several components in a step-by-step arrangement that are movable in order to shift the fuel from one part to another. Such an application allows easy transition of the fuel in the combustion chamber.